Astronomers may now fully understand why the sky is dark at night

Each time we gaze up at the night sky, we take for granted that
it's a pitch-black void studded with twinkling stars.

This is called Olbers' Paradox, and though the idea was posed
in the 1500s, it still remains somewhat controversial.

It asks the question: If the universe is essentially infinite and
ageless, why don't we see a uniform field of stars — and a
blindingly bright sky of visible light instead of a dark one?

Modern explanations suggest the universe is finite, has an age,
and is expanding faster and faster, which (according to
a discovery by Edwin Hubble in 1929) shifts the light of the
most distant stars to colors that human eyes can't see.

We've also since discovered the cosmic
background radiation, which does illuminate the sky
uniformly, and in a few different wavelengths of light. We can't
see that energy with our eyes, so we built sensitive instruments,
like the
COBE satellite, to detect it for us.

Paradox solved — mostly.

There could be another piece of the puzzle, according to a
new study in The
Astrophysical Journal, and astronomers may finally be able to
fully put the centuries-old question to rest.

The new study checked that figure by estimating the density of
galaxies from close by all the way to the farthest edges of the
universe that we can see. Because the speed of light is finite —
and can take billions of years to reach Earth — as they
looked farther out, they also looked back in time toward the
cosmos' youngest eras.

The team of four astronomers, led by Christopher Conselice at the
Leiden Observatory in the Netherlands, began by reprocessing
photos of the deepest, darkest patches of space.

That data included an ultra-deep photo taken by NASA's Hubble
Space Telescope, which reveals galaxies that existed when the
universe was as young as
400 to 700 million years old. (The universe from our vantage
point is 13.8 billion years old.)

They counted galaxies in multiple wavelengths, charted them
in three dimensions, and figured out how many there were at
various distances and epochs of time:

They discovered the density of galaxies increased the farther
back in time that they looked. This made sense, since galaxies
regularly merge and grow larger over time, and they were looking
at earlier eras. (Our
Milky Way galaxy, for example, is
on a collision course with the nearby Andromeda galaxy.)

But the density of galaxies went up only up to a certain point —
then fell off.

"[T]hese observations do not reach the faintest galaxies," the
authors concluded, adding: "we know that there should be many
more faint galaxies beyond our current observational limits."

By extrapolating the rates they saw, and assuming that something
was blocking their view, they think previous estimates of the
number of galaxies in the observable universe may be off by a
factor of 10, 20, or more.

Put another way, there are 2 trillion galaxies in the
universe instead of 100 billion.

"This question is not only of passing interest as a curiosity,
but is also connected to many other questions in cosmology and
astronomy," the team wrote in their study.

What is hiding 90% of galaxies brings us back to Olbers' Paradox,
and why the night sky is dark.

The researchers say most solutions to the paradox fall into two
buckets: one, they explain how stars and galaxies vanished; or
two, they explain why a lot of stars and galaxies are out there
but can't be seen from our earthly vantage.

The most popular idea is a bit of both. It suggests that an
expanding universe has red-shifted galaxies out of view, combined
with the facts that the universe has a finite age and an
observable size.

But Conselice and his colleagues went a step further and added
another answer to the riddle of why there isn't a similar
background glow for visible light, especially with all of these
newly discovered galaxies.

They
suggest that absorption of light by gas and dust that's drifting
through space — a long-discarded piece of Olbers' paradox, which
was originally thought to make the bright-sky problem worse — is
playing a darkening role.

The old rationale was that an infinite field of stars would
infinitely heat up the gas and dust until it, too, was as bright
as a star.

But the authors suggest that distant and red-shifted (though
otherwise visible) galaxies could have their light absorbed by
gas and dust in the void of space, then re-emitted in infrared
and ultraviolet wavelengths that are invisible to human eyes.

"It would thus appear that the solution to the strict
interpretation of Olbers' Paradox, as an optical light detection
problem, is a combination of nearly all possible solutions —
redshifting effects, the finite age and size of the universe, and
through absorption," the researchers wrote.

In the next 10 years or so, as
bigger and
more sensitive telescopes on the ground and in space go
online, the team hopes to take advantage of the deepest images of
space ever made, and in wavelengths the human eye can't see, to
test if their hunch pans out.

"It boggles the mind that over 90 percent of the galaxies in the
universe have yet to be studied," Conselice said in
a NASA press release.